Coating compositions



rimmed Mar. 7, 1939 UNITED STATES COATING COMPOSITIONS Oswald H. Greager and Gordon D. Patterson, Wilmington, Del., assignors to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing. Application May 15, 1935, Serial No. 21,693

16 Claims. (Cl. 134-39) This invention relates to coating compositions, and more particularly to rust-inhibitive priming compositions for ferrous metal surfaces.

The rust-inhibitive chromate pigments are well known to the art and these pigments have been widely used in anti-corrosive priming compositions. Although they are expensive and in some cases show undesirable settling and caking tendencies in ready mixed paints, the use of auxiliary pigments in efforts to overcome these defects has been held to a minimum in quality products because of the definite lowering in anticorrosive efliciency resulting therefrom. Certain fibrous extender pigments, for example, which have the valuable property of retarding settling in the can, are considered adulterants if employed in amounts exceeding relatively small percentages of the total pigment because of the deleterious effect on the quality of the product. Tests have shown that replacement of half the basic lead chromate in a paint by fibrous magnesium silicate, a commonly used anti-settler, markedly reduces the corrosion resistance of that system. Other cheap extenders in ordinary use, such as china clay, have the same effect.

This invention has as its object the production of new and useful metal protectivepaints. A further object is to improve upon the effectiveness of standard rust-inhibitive chromate pigments so as to secure the maximum in rust resistance from oil-type coating compositions. A further object is to improve the settling and oaking tendencies of such pigments in ready-mixed paints, this improvement being secured with no sacrifice in anticorrosive eflicency. 'A further object is to prepare highly effective primer coatings of outstanding flexibility and durability. A still further object of this invention is the manufacture of new and useful pigment compositions. Other objects will appear hereinafter.

Flake extender pigments have been used in small percentages in some types of compositions to lower cost, add bulk and in some cases to alter properties such as sanding, but the advantages disclosed herein of using large amounts of flake extender pigments in rust-inhibitive priming compositions have, however, not heretofore been recognized and insofar as we are aware no compositions of this kind have been prepared containing flake extender pigments in amounts which we have found to be effective for the purposes of the present invention. Flaked pigments, other than extender pigments, such as aluminum, have of course been used alone for decorative and protective purposes.

We have discovered, as more fully pointed out hereinafter, that flake extenders, even when they possess the same chemical constitution as the non-flake extenders used for the prior. art purposes mentioned above (as' for instance flbrous magnesium silicate and china clay), when used in large amounts markedly improve instead of lower the degree of protection obtained from the standard rust-inhibitive pigments.

The term flake extender is used herein in the usual sense as referring to a non-metallic extender pigment in which two dimensions are considerably greater than the third. As is commonly understood, extender pigments are those substantially inert pigmenting materials having a so refractive index so low that appreciable hiding power is not imparted to the paint film.

The above mentioned objects are accomplished by formulating paints comprising vehicle (preferably of the oil type) rust-inhibitive chromate pigment and flake extender in an amount not less than 15% by volume of flake in the total paint solids. We prefer, as a rule, to use a pigment mixture containing at least by volume of flake extender. Thus, a preferred formulation comprises by volume about vehicle, 20% flake extender, and 15% rust-inhibitive chromate pigment.

By oil-type vehicle we mean either drying oils themselves, varnishes (i. e. heat-blended mixtures of resins and drying oils) and oil modified polyhydric alcohol-polycarboxylic acid resins. The film-forming ability of these additional vehicles is due in large part to the oxidation of the drying oil acid radicals, thus making them closely related to the oils.

The metal protective paints which are improved by the present invention are those formulated with the chromate pigments well known to the trade as being rust-inhibitors. ments are zinc chromate, basic lead chromate, and mixed chromates of zinc and lead.

As previously indicated, the flake component should not be substantially less than the amount Such pigpreviously mentioned. When substantially small- 50 er amounts are used, the effect produced by the flake is secondary in importance and relatively slight improvement is obtained over unmodified products. It is desirable to use still greater proportions of flake extender than the minimum mentioned above, but an important limiting condition is the loss of hiding power accompanying such use. Thus, in the majority of cases, 75% by volume of flake on the total pigment represents the practical maximum, this amount of flake corresponding to a minimum volume of 25% for the rust-inhibitive pigment when, for maximum effectiveness, no third pigment is present. In certain combinations, however, as much as is useful to secure optimum results, in which case the remaining 10% must consist wholly of rust-inhibitive pigment. This improvement in rust-inhibitive effect by the presence of such relatively large amounts of the flake extender is quite unexpected in view of the fact that increase in the small proportions of nonflake extender which have been used for cheapening or increasing bulk markedly reduces corrosion resistance.

Example I Weight Volume Pigment:

Zinc chromate 50 47 Flake talc (diameter 10-20 microns) 50 53 100 Vehicle ingredients:

Oil modified resin 6L6 Mineral spirits 66.5 Cobalt linoleate-.. 0. 3 Lead linoleate. 8.0 136. 4

Total 236. 4

The resin in the foregoing example is an oil modified polyhydric alcohol-polycarboxylic acid resin prepared by heating together in the usual manner 11.5 parts glycerol, 30.2 parts linseed oil acids, and 1 9.9 parts phthalic anhydride until the proper degree of resiniflcation is secured (acid It will be understood, from the presence of iron oxide in Example 11 above, that the com posite pigment may contain other pigments in addition to the rust-inhibitive and flake pigments, it only being necessary that the latter two pigments be present in the amounts previously stated, i. e., 10% or more by volume of rust-inhibitive pigment and 50% or more by volume of flake pigment.

The extender and rust-inhibitive pigments, if desired, may be previously blended for subsequent manufacture of the priming compositions. While we do not wish to restrict ourselves to a particular method of blending, one convenient process for preparing such composite pigments is to .strike the chromate pigment in the presence of flake extender. That is, flake talc or mica or similar flake material is suspended in one of the solutions used in the chromate strike, with the result that the precipitated pigment is formed in intimate contact with and at least partially coalesced on the flake particles. Such a method is illustrated in the following examples:

Example IV 200 parts of lead-free zinc oxide are slurried in 1000 parts of water. 400 parts of flake talc are then added, followed by 92 parts of 66 B. sulfuric acid. The temperature is adjusted to 50 C. A solution of 200 parts of potassium dichromate in 500 parts of water is then added with good stirring and the stirring continued for several hours. The pigment thus formed is filtered oif, washed, dried and disintegrated.

Example V 450 parts of lead acetate are dissolved in 2000 parts of water. 225 parts of flake talc are then slurried into this solution and the temperature raised to 80-90 C. (solution A). parts of normal potassium chromate are dissolved in 2000 parts of water and the solution heated to 80-90 C. (solution B). Solution B is then run into solution Awith good stirring. The mixture is allowed to stand 1 hour and after this time the clear top liquor drawn oil as far as possible. A solution of 70 parts of sodium hydroxide in 400 parts of water, heated to 80-90 C., is then added to the above slurry with agitation and the whole boiled for 15 minutes. After cooling the resulting mass, the pigment is washed four times by decantation and finally flltered, washed, dried and disintegrated.

Other methods may also be employed to prepare our blended pigments, and useful products are obtained when the flake extender is incorporated at any convenient stage in the chromate pigment manufacture, or by subsequent wet or dry mixing of the ingredients. For example, the extender may be added to the finished strike, during the washing operation or at some point in the drying and classification process. The only requisite is that the flake structure is not destroyed by the method used.

The flake extenders used in the practice of the present invention are the non-metallic flake extender pigments, examples of which are talc, pyrophyllite, and such micas as muscovite, paragonite and lepidolite. These materials furthermore have the optimum particle size of 4-50 microns in diameter and have a thickness of about one micron. The diameter and thickness of ordinary pigments are both in the neighborhood of one micron. Various natural and synthetic inorganic minerals which possess the requisite flaky physical structure may be used. Within the optimum particle diameter range of 4-50 microns, the range of 4-20 microns is particularly desirable in those cases in which film texture is critical. Smaller particles approach an approximately cubic or spheroid shape; larger sizes, ranging up to 150-200 microns, can be used but are less satisfactory.

The vehicle or binder for the compositions disclosed herein may be any of those used by the trade for compositions of similar type. These binders include resins, varnishes, and treated or untreated drying oils. Among the suitable drying oils are linseed oil, tung oil, soya bean oil, perilla oil, oiticica oil, saiiiower seed oil. Varnish resins for suitable varnishes include natural gums such as Congo, East India, kauri, Manila, and pontianac; synthetic gums include ester gum and the amberols as well as straight phenol-formaldehyde condensation products. Oil modified polyhydric alcohol-polycarboxylic acid resins based on the drying oils mentioned above are representative of suitable vehicles in this classification. Preferred Vehicles are the oil modified polyhydric alcoholpolycarboxylic acid resins and phenolformaldehyde resin varnishes containing 45-70% oil, the former being particularly valuable because of their superior durability characteristics. Other oil-gum varnishes containing 78-86% oil (45-75 gallon oil length) also give highly efi'ective coatings. In this case the longer oil products are best obtained by blending a varnish with bodied oil, as moisture impermeability is thus retained to a higher degree. The longer oil products can also be made by blending the resin directly with the required amount of oil.

The total pigment volumes of the dried films, that is the total pigment volume based upon the total paint solids, from the compositions of mixture itself contains in minimum amount 10% of the rust-inhibitive pigment.

The rust-inhibitive coating compositions or paints of this invention may be prepared by any of the usual methods; for example, grinding of the vehicle with the pigment in a ball mill, colloid mill, or the like. Solvent and additional vehicle if desired may be added to the resultant mix in order to produce a paint of the desired consistency. Other methods known to the art may also be used.

Our improved compositions are used to most advantage as priming coats in metal protective finishing systems. They are adaptable not only to normal outdoor exposure conditions, as in the case of bridges, tanks, railway structures, etc., but also to the more severe corrosive conditions represented by underwater exposure, seashore and industrial (acid) atmospheres. They are designed primarily for the prevention of corrosion on fer-' rous metal surfaces, but need not necessarily be limited to this one type of metal, as they are also eifectivein protecting light weight alloys such as are used in airplane manufacture.

The superiority of the present primer compositions over those of the prior art has been demonstrated in outdoor exposure tests. A similar superiority under conditions of continuous water immersion is indicated by their greater water excluding properties, as determined by direct test of water permeability on detached paint films.

In addition to the advantages in corrosion resistance the flake extender pigment combinations described herein are characterized by good can stability in ready-mixed paints. The chromate pigments of this invention frequently tend to settle and cake hard in the can, and it is common practice to incorporate some fibrous extender to improve the settling properties, even at the expense of a certain degree of rust resistance (ordinary extenders invariably reduce the protective value of the prime pigment). Flake extenders, on the other hand, not only improve the protective properties of chromate pigments, but due to their physical structure also prevent the hard caking of the latter.

Another advantage of this invention lies in the improved flexibility retention of primer films. With reactive pigments, there is considerable embrittlement and hardening of the film, which renders it less adaptable to the expansion and contraction of the metal substrate under temperature changes. Flake extenders, however, do not harden the film, and the net result of using a large proportion of such pigment to replace the usual products is to obtain a much more flexible and crack-resistant coating.

The present invention has also an important advantage from the standpoint of cost, as flake extenders are relatively inexpensive. When substituted for the heavy chromate pigments on a volume basis, the savings in ingredient cost amount to a considerable proportion of the total. The ratio of unit volume costs for talc and zinc chromate or basic lead'chromate is, for example, approximately 1/ 10 at present prices.

It will be seen from the foregoing that we have by means of the invention described herein brought about a remarkable improvement in rust resistance over the standard chromate paints, which are accepted by the art as representing most eifective compositions for priming metal surfaces.

As many apparently widely different embodiments of this invention may bemade without departing from the spirit and scope thereof, it is to be understood that we do not limit ourselves to the specific embodiments thereof except as defined in the appended claims.

We claim:

1. A metal-protective coating composition comprising 50% to by volume of an oil type vehicle, rust-inhibitive chromate pigment in amount of at least 2% by volume, and flake extender in amount not less than 15% by volume,

all of said amounts being based upon the total paint solids.

2. A mixed pigment comprising at least 10% by volume of rust-inhibitive chromate and at least 50% by volume of flake extender.

3. The composition set forth in claim 1 in which the rust-inhibitive chromate pigment is zinc chromate.

4. The composition set forth in claim 1 in which the rust-inhibitive chromate pigment is basic lead chromate.

5. The coating composition set forth in claim 1 in which the vehicle is an oil-type vehicle and in which the particle size of the flake extender is 4 to 50 microns in diameter and about one micron in thickness.

6. The coating composition set forth in claim 1 in which said vehicle comprises oil modified polyhydric alcohol-polybasic acid resin.

7. The coating composition set forth in claim 1 in which said extender is flake talc.

8. A mixed pigment comprising at least 25% by volume of rust-inhibitive chromate and at least 50% by volume of flake extender.

9. The mixed pigment set forth in claim 2 in which the rust-inhibitive pigment is zinc chr0- mate.

10. The mixed pigment setforth in claim 2 in which the rust-inhibitive pigment is basic lead chromate.

OSWALD H. GREAGER. GORDON D. PATTERSON. 

